Development and use of radiation detection technology for buried seabed particles

From the initial 1997 diver-based gamma survey of the seabed sediments offshore of Dounreay, there has been continuous development of instrumentation and techniques. The initial contract surveyed randomly chosen areas, to ascertain if any particles were indeed present, since they had been found on the Dounreay foreshore in previous years. In total 34 particles were located and recovered. The period 1998-2002 saw further diver-based surveys using more sensitive and better designed detection systems. A towed system incorporating the same detectors was also deployed, covering extensive areas of the seabed. Throughout this period a more detailed understanding of particle dispersion emerged. The primary source of particles was identified as the old diffuser, with evidence for a dispersion plume heading north-east. In late 2002, Fathoms selected a gamma spectrometry system for trial and evaluation for possible future subsea deployment. The positive results led to a field trial being awarded by UKAEA for deployment of a stationary platform with a 7.8 cm x 7.8 cm NaI detector on the seabed at various offshore locations. This trial identified particles by their 137Cs photopeak and delivered the explanation for the gamma activity banding in the 'anomalous' zone. This successful trial led in 2004 to a joint Fathoms/UKAEA lab trial of the SAM-935 system of the larger 10 cm x 10 cm x 40 cm NaI crystal, inside a marinising unit. These proved to be fit for purpose and UKAEA tasked Fathoms to deliver in 2004 a tracked remotely operated vehicle (ROV) capable of deploying the larger detectors to allow gamma mapping of seabed sediments, up to a maximum depth of 100 m. Preliminary results of the 2005 ROV work are presented.